A conventional method for detecting thermal neutrons is based on detection of the effects of secondary charged particles produced when a thermal neutron is captured by a 3He nucleus. This reaction results in the production of a 3H nucleus with a kinetic energy of 190 KeV and a proton with a kinetic energy of 570 KeV. These energetic charged particles produce ionization tracks in surrounding substances. The ionization track will include ionized gas molecules (ions) which can be detected either by optical emissions or by direct collection of ions. Optical detection has an advantage over ion collection of more rapid response time and insensitivity to noise caused by vibration.
A continuing need exists for the development of improved neutron detection systems capable of offering (a) better collection efficiency than standard 3He tubes, (b) better ruggedness and/or compactness for portability applications, and/or (c) the determination of both the energy of the neutron and the direction of travel. Various attempts to achieve one or more of these and other goals are described in the below-summarized patents.
U.S. Pat. No. 5,155,366 discloses an apparatus and method for detecting a particular type of particle (e.g. neutrons) in an energy range of interest. The apparatus includes two PMTs which are spaced apart in facing relation to one another. A scintillator, positioned between the PMTs, comprises an array of optical fibers arranged substantially contiguously side-by-side. Each of the fibers has a first end proximate the one PMT and an opposing end proximate the other PMT. Also, each fiber has one of its ends being non-transmissive of light, and the fibers are arranged so that contiguous ones do not have their same ends being non-transmissive of light. Each of the fibers has a cross sectional dimension chosen in relation to a distance that the particular type of particle in the energy range of interest can travel. A signal processor unit discriminates between different types of particles and rays by determining the number of fibers affected within a predetermined time interval by an incoming particle or ray.
U.S. Pat. No. 5,231,290 discloses a neutron detector that relies upon optical separation of different scintillators to measure the total energy and/or number of neutrons from a neutron source. In a pulse mode embodiment, neutrons are detected in a first detector which surrounds a neutron source and in a second detector surrounding the first detector. An electronic circuit insures that only events are measured which correspond to neutrons first detected in the first detector followed by subsequent detection in the second detector. In a spectrometer embodiment, neutrons are thermalized in the second detector which is formed by a scintillator-moderator and neutron energy is measured from the summed signals from the first and second detectors. No directional information is provided.
U.S. Pat. No. 5,410,156 (RE36,201) discloses a fast neutron x-y detector and radiographic/tomographic device utilizing a white neutron probe. The detector detects fast neutrons over a two dimensional plane, measures the energy of the neutrons, and discriminates against gamma rays. The detector face is constructed by stacking separate bundles of scintillating fiber optic strands one on top of the other. The first x-y coordinate is determined by which bundle the neutron strikes. The other x-y coordinate is calculated by measuring the difference in time of flight for the scintillation photon to travel to the opposite ends of the fiber optic strand 20. Neutron energy is calculated by measuring the flight time of a neutron from a point source to the detector face.
U.S. Pat. No. 5,289,510 discloses a nuclear reaction detector with optical fibers arranged in side-by-side relationship in X and Y directions with a layer of nuclear reactive material operatively associated with surface regions of the optical fiber arrays. This arrangement provides position sensitivity with submillimeter resolution in two dimensions.
U.S. Pat. No. 5,880,469 discloses an apparatus and method for discriminating against neutrons coming from a direction other than a preferred direction and for discriminating against gamma rays. The apparatus includes two photomultiplier tubes that are parallel to each other and are attached to one end of a light pipe. A neutron scintillator is attached to the other end of the light pipe. The scintillator is comprised of optical fibers arranged contiguously along a first direction, which is perpendicular to a length dimension of the optical fibers, and which optical fibers alternate between optical fibers which emit photons only in the lower portion of the electromagnetic spectrum and optical fibers which emit photons only in the higher portion of the electromagnetic spectrum. Two filters are provided between the PM tubes and the light pipe, one filter transmitting only photons in the lower end of the electromagnetic spectrum and the other filter transmitting only photons in the higher portion of the electromagnetic spectrum.
U.S. Pat. No. 5,519,226 discloses an apparatus for detection of thermal neutrons including a volume of gas which includes 3He. A wavelength shifting optical (WSO) fiber is disposed to receive ultra-violet photons generated by reactions between neutrons and 3He. UV photons are absorbed within the WSO fiber to produce longer wavelength fluorescence generated photons that propagate within the WSO fiber. A photodetector is disposed to receive fluorescence generated photons from at least one end of the optical fiber and provide an output signal corresponding to neutron detection.